1. Field of the Invention
The present invention relates to a fuel cell, and in particular, to a technology for cooling the fuel cell using a separator interposed between unit cells of the fuel cell.
2. Description of the Related Art
A unit cell of a PEM fuel cell, which is one type of fuel cell, includes a fuel electrode (also referred to as the “hydrogen electrode” since hydrogen gas is generally used as the fuel), an oxidant electrode (likewise, hereinafter referred to as the “air electrode” since air, as an oxygen-containing gas, is used as the oxidant), and a solid polymer electrolyte membrane interposed between the fuel electrode and the air electrode. The fuel electrode and the air electrode each include a catalyst layer containing catalyst substance and an electrode base member. The electrode base member functions to support the catalyst layer, and allows reactant gas to flow therethrough. Further, the electrode base member also acts as a current collector. Separators (connector plates) are stacked outside the fuel electrode and the air electrode. Gas flow fields are formed on the separators for uniformly supplying the hydrogen and the air as reactant gases from the outside of the cell to the electrode surfaces, and discharging excess reactant gases to the outside of the cell. Generally, the gas flow fields are formed from grooves facing the electrode surface. The separators prevent gas leakage, and enable generated electrical current to be collected and transferred to the outside. The unit cell and the separators form a single unit cell.
In an actual fuel cell, a large number of the unit cells are stacked together serially to form a cell module. In the fuel cell, in order to maintain sufficient power generation efficiency, it is necessary to maintain the humidity of the solid polymer electrolyte membrane at a sufficient level. In general, since the water generated in the electrochemical reaction is not sufficient for maintaining adequate moisture, it is necessary to provide a device for supplying water to each of the unit cells for humidification. Further, since heat energy is also generated (the amount of this heat energy substantially corresponds to the electrical energy generated in the electrochemical reaction), a cooling device for preventing overheating of the fuel cell body is provided.
Various types of cooling devices for the fuel cell have been proposed. In some types of cooling device, cooling is performed together with humidification of the electrolyte membrane (for example, see Japanese Patent Application Laid-Open No. 10-247505). In this technology, water is added to the air supplied to the fuel cell in advance. The water is vaporized in a cooling gas flow field so as to cool the fuel cell, and then the air containing the water vapor is circulated through an air flow field.
Further, in another type of proposed cooling device, a hollow space separated from a gas flow field is formed in a separator. Cooling water flows through the hollow space, and vapor of the cooling water is supplied to an air flow field through a porous wall surface (for example, see Japanese Patent Application Laid-Open No. Hei 06-338338).
However, with the related disclosed technology, it is difficult to perform both cooling and humidification of the membrane. For example, according to the disclosure of Japanese Patent Application Laid-Open No. 10-247505, the liquid water is vaporized into vapor in the cooling gas flow field, and the air containing the vapor is then circulated again into the air flow field. As a result, it is difficult to maintain the temperature of the cooling gas flow field in the circulation passage. For example, when the temperature increases in the air flow field having decreased in the circulation passage, moisture is removed from the electrolyte membrane, which makes it is difficult to maintain the humidity in the membrane.
Further, with the technology disclosed in Japanese Patent Application Laid Open No. 06-338338, even though the vapor is supplied through the porous wall surface, it may be difficult to achieve sufficient supply of vapor by using the moisture passing through the porous wall surface. Further, in the cooling water passage, cooling is only performed using the sensible heat. Therefore, in order to perform sufficient cooling, substantial amounts of energy or a mechanical device for circulation of the cooling water may be required.